The Gm-C filter is a common filter structure with low power consumption and high bandwidth and is applicable to various wired and wireless technology fields.
The most obvious weakness of an on-chip integrated filter is the strong process dependency between the integrated capacitance and resistance. This always results in a large offset of the cut-off frequency of the filter from the designed value and affects the filter performance. Therefore, as the on-chip integrated filter is put in use, a tuning circuit is necessary for the filter. In a Gm-C filter structure, for tuning of the filter frequency (the quality factor Q of the filter also needs to be tuned in some occasions), Gm tuning is generally adopted.
Because the cut-off frequency of a filter is in direct proportion to Gm/C, filter frequency tuning is always implemented by changing the Gm value. FIG. 1 shows the structures of two common Gm units, MOS input (or diode input) and degradation resistance input. An obvious characteristic of the two Gm units is that the Gm value is not in absolute proportion to the current intensity or resistance and therefore it is hard to control the cut-off frequency of the filter through discrete currents. As a result, tuning based on a digital Phase-Locked Loop (PLL) structure is popular.
FIG. 2 shows a filter frequency tuning process using a PLL to regulate the tail current. A PLL is integrated on the chip and the PLL utilizes a Voltage Control Oscillator (VCO) made up of Gm-C units that have the same structure as the filter to implement frequency control. The PLL decides the frequency control of the VCO by regulating the tail current of the VCO.
During the implementation of the present invention, the inventor finds that the filter frequency tuning solution in the prior art has the following weakness:
The frequency tuning is implemented via the change of the tail current of the VCO. As a result, the linearity of the filter changes with the tail current of the VCO and thus the performance of the Gm-C filter is affected.